This invention relates to an electrostatic chuck for holding a semiconductor wafer in a fixed plane relative to said chuck, which chuck comprises an electrically conductive member separated from said fixed plane by a layer of dielectric material, means for electrically contacting the wafer, and means for supporting the wafer in said plane.
In the manufacture of semiconductor devices it is sometimes necessary to clamp a semiconductor wafer substantially flat against a support so that one of its surfaces can be subjected to a processing treatment. Such a treatment may involve directing charged particles towards the wafer. For example, selected areas of the wafer can have their conductivity type modified by the implantation of ions. As another example, an electron sensitive resist may be coated on the wafer surface and the resist can then be selectively exposed to a beam of electrons. During these processing treatments it is important that the wafer be held flat against the support. For this purpose it is known to use a so-called electrostatic chuck.
United Kingdom Patent Specification GB No. 1,443,215 describes an electrostatic chuck which essentially comprises a substantially flat electrically conductive support member coated with a layer of dielectric material. A semiconductor wafer can be supported on the dielectric layer which thus prevents physical and electrical contact between the facing surfaces of the wafer and the support. The chuck also has means for electrically contacting the wafer so that a potential difference can be applied between the wafer and the support. Such a potential difference sets up an electrostatic clamping force across the dielectric layer so that the wafer is then held substantially flat against the dielectric layer. With the wafer thus clamped, its surface remote from the support can be subjected to the appropriate processing treatment.
Unfortunately, processing treatments involving the use of beams of charged particles as mentioned above are responsible for the generation of thermal energy in the wafer. This thermal energy can cause expansion and local distortion of the wafer if the heat generated cannot be readily dissipated. The chuck described in GB No. 1,443,215 relies on firm and even clamping of the wafer to obtain good heat exchange between the wafer and the support. However, the wafer is actually clamped against the dielectric layer which separates the wafer from the support. In general, the thermal conductivity of dielectric materials is not particularly high. Thus the dielectric layer, which is necessary for electrostatic attraction, acts as a barrier to the efficient flow of heat from the wafer to the support so that it is necessary to use thin layers of high thermal conductivity dielectric in order to obtain adequate heat conduction. It can be said, therefore, that for adequate clamping and good heat transfer between the wafer and the support, the chuck described in GB No. 1,443,215 must have a thin layer of high thermal conductivity dielectric material with a high dielectric constant and a high dielectric strength. Clearly the choice of materials and design flexibility is severely restricted by these requirements.